The present invention relates to plasma working of electrically-conductive materials, and more specifically to a method for striking a main arc between an electrode of a plasmatron and workpiece, and apparatus embodying same.
The preferable field of application of the invention disclosed is plasma working of electrically-conductive materials in oxidizing plasma-forming media with the use of electrodes based on zirconium or hafnium as, for example, manual air-plasma cutting of metals by means of an air-cooled plasmatron.
Furthermore the present invention may find application in plasma working of electrically-conductive materials in a variety of chemically-active plasma-forming media such as hydrocarbons and gaseous by-products of the processes of carbonising coal and melting metals, etc.
There is widely known in the art a method of striking a main arc between the electrode of a plasmatron and a workpiece. In the known method, the process of striking a main arc falls into two phases. Initially, a high-voltage discharge is brought about between the electrode and nozzle of a plasmatron with the aid of an oscillator to initiate a pilot arc whose cathode and anode spots are located on the electrode and nozzle while a flow of ionized gas, i.e., the visible flame of pilot arc, is issuing from the passage in the nozzle throughout the entire period of the arcing of pilot arc. At the next stage, provided the flame reaches the surface of the workpiece and the potential applied between the electrode of the plasmatron and workpiece is sufficiently high to sustain an electric arc, the main arc is struck between the electrode of the plasmatron and the workpiece.
The process described is embodied in a known apparatus which includes a d.c. power source one of the whose terminals is connected to the electrode of a plasmatron provided with a nozzle insulated from the electrode and with means of feeding a plasma-forming gas while the other terminal is connected to the workpiece through a main arc current transducer. In the known apparatus, the nozzle of the plasmatron is connected to a d.c. power source in series with a resistor and a switching device which form the circuit of the pilot arc. Interposed into the circuit between the nozzle and electrode of the plasmatron there is also an oscillator.
The known apparatus provides for a reliable striking of a pilot arc arcing unceasingly between the electrode and nozzle of the plasmatron until the main arc is struck. When atomic gases are being used as the plasma former, no thermal deterioration of the nozzle takes place even if the current of the pilot arc arcing during a protracted period is of a high magnitude. Thus the known apparatus has come into widespread use when an atomic gas, as argon for example, is used as the plasma-forming medium.
Yet if a molecular gas is employed in the capacity of plasma-forming medium, the known apparatus is of little help. The point is that in apparatus for plasma working by means of an external arc produced in a molecular gas when the length of nozzle passage is an inherently limited one, the anode spot of pilot arc shifts itself to the end face of the nozzle, bringing about an erosion of the nozzle at a high rate and destroying the nozzle. Furthermore, the current of pilot arc can practically be not lower than 60 to 80 A in this case because an attempt to lower the current results in a sharp increase in the floating voltage of power source up to a level hazardous to the personnel, particularly if the job entails manual plasma working. On the other hand, if the current of the pilot arc arcing unceasing in an atmosphere of molecular gas is maintained at said level, this brings about rapid thermal deterioration of a water-cooled nozzle and an almost instantaneous ruining of a gas-cooled one.
Known in the art is apparatus for welding and cutting with the aid of a plasma jet (see U.S. Pat. No. 3,174,027 of Mar. 16, 1965, by the same authors and German Pat. No. 1,236,692 granted in the Federal Republic of Germany on Mar. 16, 1967) wherein for initiating a main arc between the electrode of plasmatron and the workpiece there is struck repeatedly and for a short while a pilot arc, the latter being turned off while the main arc is arcing. Serving this purpose is a known RC-curcuit or a switching device in the circuit of the pilot arc ("Electrooborudovanie novoi universalnoi ustanovki dlya gascelectricheskoi reski metallov" by D. C. Bykhovsky and Yu. A. Bogorodsky, "Electropromyshlenoost i priborostroenie", No. 22, 1960).
Said method and the apparatus embodying same eliminate the thermal erosion of the nozzle and provide for a failproof striking of the main arc in atomic gases, the distance between the plasmatron and workpiece being within the range of 5 to 10 cm in this case.
However, studies have revealed, on one hand, that under the conditions when a molecular gas or a mixture of molecular gases is used for plasma forming said apparatus for striking a main arc fails to assure failproof closing of the circuit between the electrode of the plasmatron and workpiece by means of a flow of ionized gas (flame of pilot arc) coming into being due to a current passing through the circuit of pilot arc, on the other hand, which is the main point, some of the current pulses in the circuit of the pilot arc. viz., those passing at a low on-off time ratio, fail to heat the active surface of the electrode where the cathode spot of arc is located to a temperature required in operation.
It is thus obvious that the use of chemically active gases and mixtures thereof as the plasma-forming medium practically brings to nothing all the advantages offered when a main arc is being struck by means of a periodically interrupted pilot arc because in this case the known method and apparatus embodying same arc is not capable of providing for a failproof striking of the main arc.
It is an object of the present invention to improve the reliability of striking a main arc in molecular gases.
Another object of the present invention is to obtain a continuous visually-monitored flame of a pilot arc of considerable intensity for striking a main arc in a medium of chemically active gases under the conditions of high rates of gas flow.
A further object of the present invention is to increase the distance between the electrode of the plasmatron and workpiece at the instant of striking the main arc.
These and other objects are attained by the fact that in a method for striking a main arc between the electrode of a plasmatron and workpiece wherein an intermittently-arcing pilot-arc is initiated by a high-frequency discharge and the flame of the pilot arc bridges the gap between the electrode of the plasmatron and workpiece due to the influence of a gas fed into the plasmatron, making thereby the circuit of main arc, the amplitude of the pilot arc current is selected in accordance with the invention so that it is roughly equal to the steady current of the main arc and the frequency and on-off time ratio of the current are selected so as to assure that the flame of the pilot arc is sustained unceasingly throughout the arcing of the visually-monitored pilot arc.
In a method for striking a main arc between the electrode of a plasmatron and the workpiece it is expedient that the amplitude of the pilot arc current is selected with in the range EQU i .perspectiveto. I - 3I,
where
i = amplitude of pilot arc current;
I = steady current of main arc;
and the frequency and on-off time ratio of the current are selected so as to assure that the average current in the circuit of pilot arc is between 3 and 10 A throughout the arcing of the arc.
In a method for striking a main arc it is further expedient to select the amplitude of pilot arc current within the range EQU i = 0.8 I - 3 I,
where
i = amplitude of pilot arc current;
I = steady current of main arc. PA1 failproof striking of main arc in a medium of molecular gases or mixtures thereof; PA1 producing a visually-monitored unceasing flame of highintensity pilot arc which assures failproof striking of main arc in a medium of chemically active gases flowing at a high rate; PA1 cooling all parts of the plasmatron with gas so that a miniature apparatus for manual air-plasma working is a practical possibility; PA1 saving on the cost of manufacturing the apparatus for striking main arc thanks to smaller dimensions, lower weight and cost of the pilot arc-inducing circuitry; PA1 recourse to move advanced sources of power for plasma working in a medium of molecular gases or mixtures thereof (thyratrons, for example); PA1 elimination of double arcing should the electrical conditions or the conditions of gas flow sustaining the arcing change sharply, this elimination being possible due to the use of a diode conducting the current in pilot arc circuit in the corresponding direction.
It is also expedient that in a method for striking a main arc the amplitude of pilot arc current is 600 A.
In apparatus embodying the method, the electrode of the plasmatron is connected to one of the terminals of a power source which has an oscillator connected between the electrode and nozzle of the plasmatron and the other terminal of the source of power is connected to the nozzle through a switching device in the electrode-to-nozzle circuit which provides for an intermittent current in said circuit required for striking a main arc in the gap between the electrode and workpiece when a plasma-forming gas is fed between the electrode and nozzle, the electrode and nozzle being interconnected according to the invention by said switching device connected in series with an inductance.
It is preferable to use a known RC-circuit as the switching device and to connect a silicon diode between the nozzle and RC-circuit so that the diode conducts the pilot arc current.
It is also preferable to use a variable resistance R.sub.1 in the R.sub.1 C circuit and to link up the resistance with an actuator capable of changing the magnitude of the resistance and interconnected with a main arc current transducer, the minimum value of resistance R.sub.1 being preferably selected when a pilot arc is arcing and the maximum one when the main arc is arcing.
It is further preferable to connect a switch into the RC-circuit in series with the resistor R so that said switch makes the circuit when a pilot arc is arcing and breaks the circuit when the main arc is arcing.
Compared with the known method and apparatus embodying same, the method and apparatus disclosed offer a number of advantages such as: